This invention relates generally to electrostatographic printing, and more particularly concerns an apparatus for developing an electrostatic latent image.
The process of electrostatographic printing includes both electrographic printing and electrophotographic printing. Both of these processes are quite similar to one another, i.e., an electrostatic latent image corresponding to the original document is recorded on an image bearing member. Electrophotographic printing is disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1942. As described therein, a photoconductive member is charged to a substantially uniform potential. The charged photoconductive member is exposed to a light image of the original document. The light image selectively dissipates the charge in the irradiated areas and creates an electrostatic latent image on the photoconductive member. Electrophotographic printing creates an electrostatic latent image corresponding to the original document to be reproduced without the use of a photoconductive material or a light image.
Whatever method is employed in the formation of an electrostatic latent image, a viewable record thereof is usually produced by depositing toner particles thereon, i.e. the process of development. Development may be achieved by bringing the latent image into contact with the developer mix. Typical developer mixes employed in the art generlly comprise toner particles, such as heat settable colored thermoplastic particles, which electrostatically adhere to coarser carrier granules, such as ferromagnetic granules.
Various types of developing systems are employed in the art and include, amongst others, cascade development, magnetic brush development, powder cloud development, and liquid development. Magnetic brush systems achieve a high degree of uniform deposition and, therefore, are used in numerous electrostatographic printing machines. Various types of hybrid systems are also frequently employed. For example, a combination of cascade and powder cloud development is frequently utilized. One type of hybrid system wherein a cascade and powder cloud development are combined is disclosed in U.S. Pat. No. 3,470,009 issued to Gundlach in 1969. Another type of powder cloud and cascade development system is described in U.S. Pat. No. 3,799,113, issued to Whited in 1974. Both of the foregoing systems describe techniques wherein a developer mix of carrier granules and toner particles cascade in a downwardly direction between an apertured screen and an electrode. Toner particles are separated from the carrier granules and form a cloud thereof adjacent to the electrostatic latent image. Thi s cloud of toner particles thereafter develops the electrostatic latent image.
Multi-color electrostatographic printing involves the utilization of various process components to produce a series of electrostatic latent images corresponding to a particular color in the original document. In such a system, there is a requirement to develop successive partial color images. Each color image is developed with toner particles of a selected color. These powder images are then transferred to a suitable support surface, in registration with one another, to form a reproduction of the colored original document. In most high quality colored electrophotographic printing systems, magnetic brush systems are employed. In magnetic brush systems, development is of the contact type wherein a brush-like array is formed which is in engagement with the electrostatic latent image recorded on the photoconductive surface. It would appear to be highly advantageous to be able to combine a magnetic brush system with powder cloud systems so as to produce a non-contact magnetic brush system having solid area capability.
Accordingly, it is a primary object of the present invention to improve the development apparatus so as to form a non-contact magnetic brush development unit.